This invention relates to a electrophoretic medium and to a process for the production of this medium. This invention also relates to an electrophoretic display incorporating such a medium. More specifically, this invention relates to a two-phase electrophoretic medium which comprises a discontinuous phase containing electrophoretically-mobile particles and a continuous phase essentially free from such particles. The present application also describes materials useful in fabricating such media and displays.
Electrophoretic displays have been the subject of intense research and development for a number of years. Such displays can have attributes of good brightness and contrast, wide viewing angles, state bistability, and low power consumption when compared with liquid crystal displays. (The terms “bistable” and “bistability” are used herein in their conventional meaning in the art to refer to displays comprising display elements having first and second display states differing in at least one optical property, and such that after any given element has been driven, by means of an addressing pulse of finite duration, to assume either its first or second display state, after the addressing pulse has terminated, that state will persist for at least several times, for example at least four times, the minimum duration of the addressing pulse required to change the state of the display element.) Nevertheless, problems with the long-term image quality of these displays have prevented their widespread usage. For example, particles that make up electrophoretic displays tend to cluster and settle, resulting in inadequate service-life for these displays.
Numerous patents and applications assigned to or in the names of the Massachusetts Institute of Technology and E Ink Corporation have recently been published described encapsulated electrophoretic media. Such encapsulated media comprise numerous small capsules, each of which itself comprises an internal phase containing electrophoretically-mobile particles suspended in a liquid suspension medium, and a capsule wall surrounding the internal phase. Typically, the capsules are themselves held within a polymeric binder to form a coherent layer positioned between two electrodes. Encapsulated media of this type are described, for example, in U.S. Pat. Nos. 5,930,026; 5,961,804; 6,017,584; 6,067,185; 6,118,426; 6,120,588; 6,120,839; 6,124,851; 6,130,773; 6,130,774; 6,172,798; 6,177,921; 6,232,950; 6,241,921; 6,249,271; 6,252,564; 6,262,706; 6,262,833; 6,300,932; 6,312,304; 6,312,971; 6,323,989; and 6,327,072; U.S. patent application Publication No. 2001-0045934; and International Applications Publication Nos. WO 97/04398; WO 98/03896; WO 98/19208; WO 98/41898; WO 98/41899; WO 99/10767; WO 99/10768; WO 99/10769; WO 99/47970; WO 99/53371; WO 99/53373; WO 99/56171; WO 99/59101; WO 99/67678; WO 00/03349; WO 00/03291; WO 00/05704; WO 00/20921; WO 00/20922; WO 00/20923; WO 00/26761; WO 00/36465; WO 00/36560; WO 00/36666; WO 00/38000; WO 00/38001; WO 00/59625; WO 00/60410; WO 00/67110; WO 00/67327 WO 01/02899; WO 01/07691; WO 01/08241; WO 01/08242; WO 01/17029; WO 01/17040; WO 01/17041; WO 01/80287 and WO 02/07216.
Encapsulated electrophoretic displays solve, or at least reduce, many of the problems previously associated with electrophoretic displays, and offer additional advantages compared to liquid crystal displays. Some added advantages are the ability to print or coat the display material on a wide variety of flexible and rigid substrates. The clustering and settling problems, which plagued prior art electrophoretic displays and resulted in inadequate lifetimes for the displays are now overcome.
This application describes electrophoretic displays, especially encapsulated electrophoretic displays, and classes of materials, as well as some specific materials, which should be useful in their construction.
The successful construction of an encapsulated electrophoretic display requires the proper interaction of several different types of materials and processes. Materials such as a polymeric binder, a capsule membrane, and the electrophoretic particles and fluid must all be chemically compatible. The capsule membranes may engage in useful surface interactions with the electrophoretic particles, or may act as an inert physical boundary between the fluid and the binder. Polymer binders may act as adhesives between capsule membranes and electrode surfaces.
It has now been recognized that a separate encapsulation step of the process is not always necessary. The electrophoretic fluid may be directly dispersed or emulsified into the binder (or a precursor to the binder material—this binder or precursor may for convenience be referred to as the “continuous phase”) to form what may be called a “polymer-dispersed electrophoretic display”. In such displays, the individual electrophoretic phases (“droplets”) may be referred to as capsules or microcapsules even though no capsule membrane is present, i.e., there is no discrete capsule membrane associated with each individual droplet. Such polymer-dispersed electrophoretic displays, which are the subject of the present invention, are considered to be subsets of encapsulated electrophoretic displays.
It has now been found that two-phase electrophoretic media with advantageous properties can be prepared using a simple, inexpensive process, and this invention relates to such media and to processes for their preparation.